Electronic Theses and Dissertations (PhDs)

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    Development and Reliability Testing of a new Low-Voltage Power Supply for the ATLAS Hadronic Tile-Calorimeter Phase-II Upgrade
    (University of the Witwatersrand, Johannesburg, 2024-06) Mckenzie, Ryan Peter; Solans, Carlos; Mellado, Bruce
    The Large Hadron Collider (LHC), located at the Conseil Européan pour la Recherche Nucléaire (CERN) also known as the European Laboratory of Particle Physics, is a tworing-superconducting-hadron accelerator and collider located on the Franco-Swiss border. The LHC was successfully commissioned in 2010 for proton–proton collisions and is expected to deliver 500 f b−1 before Long Shutdown three (LS3) that is schedule to commence in 2026. Its successor, the HL-LHC, will provide a levelled instantaneous luminosity of L = 5x 1034 cm−2 s−1 and is projected to deliver an integrated luminosity of more than 4000 fb−1 to its two general purpose detectors, known as A Toroidal LHC Apparatus (ATLAS) and Compact Muon Solenoid (CMS), over a span of 10 years. The main motivation to upgrade the LHC is to fully exploit its physics potential. Through a series of machine and detector upgrades, it is possible to increase the instantaneous luminosity. This could unlock many of the physics processes that are today inaccessible to the LHC because of the lack of statistics. The primary impacts of the HL-LHC on the detector environment are a direct consequence of an increase in delivered instantaneous luminosity. The ATLAS experiment will undergo its Phase-II Upgrade during Long-Shutdown 3 to ensure peak performance during high-luminosity operations. ATLAS is composed of several specialized sub-detectors one of which is the hadronic Tile-Calorimeter (TileCal). The TileCal will undergo numerous upgrades on of which will be to the Low-Voltage (LV) power distribution system that services its on-detector electronics. The on-detector finger Low-Voltage Power supplies form the second stage of the LV system. Their primary functional device is a transformer-coupled buck converter, known as a Brick, which is responsible for converting bulk power to that required by the on-detector electronics. All legacy Bricks will be replaced with a new version that employs several design changes to enable their reliable operation within the HL-LHC detector environment. In this thesis, the development of the Phase-II Upgrade Brick is presented with an emphasis placed on its thermal performance and reliability. A thermal analysis of the proposed upgrade Brick versions is presented with design changes occurring as a result. Due to the design change incorporating a new active component an irradiation campaign is conducted to qualify it for use within the high-luminosity detector environment. A reliability analysis of the Phase-II upgrade Brick is conducted necessitated by the change of many critical components. The quality assurance procedure of the Bricks that is undertaken post-production is presented with particular attention placed on their Burn-in testing.
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    The Use of Semi-Supervised Machine Learning Techniques in the Search for New Bosons with the ATLAS Detector
    (University of the Witwatersrand, Johannesburg, 2024-06) Lieberman, Benjamin; Mellado, Bruce
    Since the completion of the Standard Model, with the discovery of the Higgs Boson, there has been a significant shift in the exploration of new physics to explain deviations between model simulated data and that produced at the Large Hadron Collider. These investigations are greatly aided by the integration of advanced machine learning techniques. Machine learning methods offer powerful solutions for complex collider physics challenges. However, these models often depend on simulations that might not fully align with actual physical phenomena. In order to remove this model dependency, semi-supervised classifiers can be used. This solution, however, is not without challenges. In this thesis, an evaluation of the use and limitations of semi-supervised classifiers in particle physics is presented. This is achieved by using a well constrained di-lepton final state dataset to assess the efficacy and ability of the technique, compared to its supervised counterpart. Following this baseline study, the S(150) ! Zg final state, motivated by the multi-lepton anomalies at the LHC, is used to perform narrow resonance searches with semi-supervised Neural Network (NN) classifiers. This work details the methodology and outcomes of a frequentist study aimed at quantifying the extent of a trials factor introduced by semi-supervised NN classifiers. This involves an in-depth analysis of the rate of statistical fluctuations generated in the training of semi-supervised techniques. A secondary component of this study is the evaluation of machine learning based data generators, with emphasis on the Wasserstein Generative Adversarial Network (WGAN), to produce large quantities of realistic data for physics analyses. The results of this investigation into semi-supervised techniques, firstly validates the efficacy and ability of these techniques to classify particle events. This is followed by the frequentist study results, which substantiation that the trials factor remains suitably managed with the use of semi-supervised NN classifiers. The insights derived from this research pave the way for enhancing the reliability of upcoming resonance searches, underscoring the potential of semi-supervised learning in searches for narrow resonances.
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    Efficiency Enhancement in Photovoltaic Devices Using Light Management and Morphology
    (University of the Witwatersrand, Johannesburg, 2024-04) Kumalo, Sandile; Quandt, Alexander; Wamwangi, Daniel
    Meeting the ever-increasing global demand for energy is society’s principal challenge for attaining economic growth and dynamic technological progress. Novel materials and technologies to extend photoabsorption and harness the solar emission spectrum are critical for producing solar-based electricity on a large scale. Current techniques and nanostructure-based approaches can revolutionise the production of solar electricity. In this work, experimental light management strategies through plasmonic nanostructures in silicon-based thin film solar cells were explored to augment power conversion efficiency (PCE). These devices incorporated plasmonic, magnetoplasmonic, and coreshell nanostructures coated with SiO2. It was demonstrated that magnetoplasmonic nanoparticles enhance interactions with both the charge of electrons and the unpaired spin with the B-field component of the electromagnetic spectrum. Furthermore, core-shell structures passivate the surface of the nanoparticles, significantly enhancing PCE. The highest PCE (10.7%) was observed for Au@SiO2 nanoparticles, attributed to a bonding plasmon mode at the interface between the nanoparticles and the surrounding bulk material. Additionally, F e3O4@SiO2 nanoparticles primarily enhanced the short-circuit current (Jsc), due to magnetic interactions with superparamagnetic nanoparticles. A detailed investigation into the Curie temperature (Tc) of various magnetic nanoparticles revealed that 4 nm F e3O4 nanoparticles possess the highest Tc of 906.1 K, indicating strong magnetic stability under operational conditions. For Ni@Fe core-shell nanoparticles, a decrease in Tc with increasing Ni content was observed, highlighting the critical role of composition in tuning magnetic properties. Morphological analysis through TEM imaging revealed uniform dispersion and spherical morphology for Au nanoparticles, crucial for consistent plasmonic properties. The addition of SiO2 shells to both Au and Ag nanoparticles significantly improved their optical absorption characteristics due to the modification of the local dielectric environment. Furthermore, a study on the bulk heterojunction of organic solar cells demonstrated that processing solvents play a pivotal role in optimising active layer performance. It was found that solvent mixtures, particularly 2-MEA and toluene in a 7:3 ratio, significantly enhance device efficiency by promoting better phase separation and charge transport, achieving a PCE of 5.77%. These findings showcase the significant potential of nanostructures and solvent processing in improving the efficiency of photovoltaic devices. The enhanced PCE and stability of devices incorporating plasmonic and magnetoplasmonic nanoparticles, along with optimised solvent processing techniques, provide valuable insights for future research and development in solar energy technologies.
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    The Low–Temperature Properties of Boron–Implanted Diamond Materials
    (University of the Witwatersrand, Johannesburg, 2024-08) Mahonisi, Nyiku Clement; Naidoo, Mervin
    The physio-chemical properties of semiconducting diamond materials under extremely low temperatures have fundamental implications in Condensed Matter Physics. Highly doped boron diamonds have been shown to reach a superconductive state at critical temperatures (Tc) ranging from 4 K − 10 K, albeit such properties are ”at the moment” only attributed to heavily boron-doped synthesized samples via HPHT and CVD growth methods. Theoretical predictions have shown that by exceeding the current solubility limit of boron in diamond, an increase in Tc beyond the 4 K − 10 K is possible, even close to room temperatures. However, in order to gain such a feat, an increase in active boron concentration beyond the metal-to-insulator transition (MIT) is an absolute necessity, and hence, non-equilibrium doping fabrication processes such as CVD growth and ion implantation are required. In this study, we explore carefully the properties of degenerate diamond layers with p-type impurity bands via low energy and low fluence ion implantation. The study involves the utilization of the non-equilibrium ion implantation technique to fabricate boron layers that stretches from the diamond surface to some depth that is a few nm deep into the diamond matrix. A total of seven samples were processed uniquely with respect to ion energies, ion concentrations and annealing temperatures. Three samples (A, B and D) with varying parameters were implanted with both carbon and boron ions in regions where the carbon distribution overlaps with the boron distribution at concentrations close to the MIT level ∼ 4 × 1020 ions/cm3. The carbon implants were used to induce vacancy defects very close to the surface such that the boron ions that are subsequently implanted would simply diffuse into the carbon vacant sites. Furthermore, four samples (BE,ME and SE) were implanted with only boron ions also with varying processing parameters to establish a correlation between the two set of implanted samples. Lastly, one sample (C) was implanted similarly to the first set of samples , albeit, with only carbon ions in order to ascertain the boron-related defects. Initially, all the samples were annealed at 650◦C in order to recover the crystalline state. Spectral analysis clearly confirms the formation of boron-related defects for the carbon and boron implants with dynamic annealing at 200◦C and at 400◦C, respectively. That is, the Lorentzian-like component ∼ 1200 cm−1 and the asymmetric line shape ∼ 1300 cm−1. However, vacancy and interstitial peaks at ∼ 1500 cm−1 and ∼ 1620 cm−1, respectively, are also prevalent. Albeit, dynamic annealing limits the graphitization of the diamond structure with no detection of graphitic features. A concentration and thermal annealing dependency was also established. The second batch of samples were multi implanted with boron (i.e., within the same buried region for samples ME and SE and overlapping implanted regions for sample BE to form a box profile). Boron-related features were not wholly detected for all these samples. After multiple implantation processes it was evident that the diamond structure of samples BE and SE were adversely affected with prominent graphitic features forming. Very faint boron-related features appear for sample ME after nine multiple boron implantation processes with low energy and low fluence at room temperature (RT). However, the spectra show the onset of broadening effects associated structural damage. Expectantly, boron-related features are absent for the carbon implanted sample C with very minimum damage to the diamond spectra. Low resistance tri-layer metal contacts of Ti/Cr/Au were deposited onto the surfaces of the samples in a 4-point probe Van der Pauw configuration. Ohmic behavior was confirmed from electronic transport measurements. Thus, the conduction properties of the samples are also reported in this thesis. A very clear dependence on the fabrication method used to create the boron buried layers is demonstrated by the electronic response of the samples. Sample A experiences Space Charge Limited Currents (SCLCs) related to high localization effects that compensate the movement of free charge carriers through the diamond matrix such that the reported data is limited to a temperature range that is between 300 K → 100 K. The carrier concentration of sample B determined from Hall effect measurements indicate a contribution of both electrons and holes, likely due to the amphoteric vacancy states induced by carbon implantation. However, a careful increase in boron fluence and thermal annealing averts such effects. Samples BE and SE showcase a high level of conductivity with variable range hopping (VRH) mechanisms at low temperatures ∼ 10 K that suggests an increase of the localization length ξloc with low TES values. The overpopulation of boron ions within the nm−sized channels of these samples results in amorphous regions that contribute to the conduction properties of the materials. Hence, a very clear difference of the conduction properties of the samples is demonstrated.
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    An unsupervised search of non-thermal diffuse emission in extended sources
    (University of the Witwatersrand, Johannesburg, 2022-01) Thwala, Siphiwe Anthony; Beck, Geoff
    Low-surface brightness diffuse radio emission is a useful probe for studying the connection between non-thermal processes in radio sources and their environment, as well as gaining insights on galaxy formation. The presence of this emission in different astrophysical and cosmological signals is not well understood in the literature due to challenges in detection caused by the low-surface brightness of diffuse objects and their origins, particularly in relation to galaxy feedback and formation. We explored the utility of publicly available data to detect, characterise, and study diffuse emission in extended radio sources at the scales of radio galaxies and galaxy clusters. We performed multiple analyses on extended radio continuum sources found in large publicly available radio surveys. The analyses range from in-depth multi-frequency examinations of individual sources to novel approaches for grouping and classifying radio sources with unsupervised machine learning. The analysis of individual sources brought together a range of datasets and techniques to provide insight about their nature. To group and classify continuum radio sources at scale, a novel unsupervised machine learning approach was designed to combine self-organising maps with convolutional neural networks for automatically detecting and clustering similar sources in radio surveys. To the best of our knowledge, this is the first implementation of an architecture that allows for the training of a machine learning model using multi-frequency and multi-scale radio continuum data cubes, as input, to automatically detect and cluster similar sources in radio surveys. This comes at a time when radio astronomy is undergoing a transformation and data mining methods are critical for optimum scientific utilisation of data from telescopes like MeerKAT, JVLA, MWA, and LOFAR (as well as the upcoming SKA). The different works showcase the most interesting radio sources with diffuse emission observed in these investigations.
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    First principle study of inorganic metal halide perovskites for solar cells application
    (University of the Witwatersrand, Johannesburg, 2024-08) Maleka, Prettier Morongoa; Maphanga, Regina R.; Ntwaeaborwa, Odireleng Martin
    All-inorganic halide perovskites have received significant attention as semiconductor materials due to their outstanding opto-electronic properties, which have achieved power conversion efficiency (PCE) of up to 25% in perovskite solar cells. Their exceptional characteristics include long diffusion lengths for electrons and holes, tuneable band gap, high absorption coefficients, small effective masses, high carrier mobility, and simple reproducible process. Despite these excellent properties, metal halide perovskites have drawbacks that negatively affect the PCE and stability of the perovskite solar cell devices. This study investigated all-inorganic halide perovskite, CsPbI3, by employing the first-principle density functional theory (DFT) method. Firstly, the effect of mixing halides on X-site was investigated to probe the structural stability and opto-electronic properties. The structural, electronic, optical, mechanical and thermodynamics properties of CsPbI3 – xBrx were investigated using three exchange correlation functionals, namely, LDA, GGA-PBE and SCAN meta-GGA. The findings revealed that mixed halide perovskites have an ideal direct energy band gap for suitable photovoltaic applications. For GGA-PBE and SCAN meta GGA exchange correlation functionals, the determined energy band gap ranges from 1.33 eV and 1.877 eV, whereas the LDA band gap ranges between 0.960 eV and 1.137 eV. The electronic band gaps predicted by GGA-PBE and SCAN meta GGA exchange correlation, which offer better precision compared to LDA suggest that Br-doped CsPbI3 – xBrx perovskite is suitable for photons absorption from near-infrared to visible regions of the spectrum. The modification of the band gap is an essential feature of photovoltaics, as it enables the optimization of solar cell performance. In addition, the systems CsPbI3, CsPbI2Br, CsPbIBr2, and CsPbBr3 exhibited exceptional mechanical and thermodynamic stability. Secondly, perovskites that are considered for photovoltaic applications contain toxic element lead (Pb) on the B-site, which limits application of these perovskites in photovoltaic devices. In this study, substitution of toxic Pb with a smaller percentage of selected transition metals was investigated in order to alleviate the toxicity problems. Thus, CsPbI3 doped with 12.5 % concentration of transition metal, Mn, Fe, Ni and Zn was investigated using DFT. The results showed that transition metal doped-CsPbI3 perovskites enhanced the absorption of this material, although they are all indirect band gaps semiconductors. All the materials were found to be mechanically stable. Lastly, cluster expansion which is a method that is capable of describing the concentration dependent thermodynamic properties of materials while maintaining DFT accuracy, was used to predict new (CsPbI/Br)3 structures. The cluster expansion method generated 42 new stable (CsPb)xIyBrz (where x = 1 to 3 and y and z = 1 to 8) structures and these were ranked the meta-stable structures based on their formation enthalpies. Monte Carlo calculations showed that CsPbI0.5Br0.5 composition separates into different phases at 300K, but changes to homogeneous phase at 700 K, suggesting that a different phase of CsPbI3 may exist at higher temperature. Among the 42 predicted structures, randomly selected structures around iodide rich, 50:50 and bromine rich sites were studied further by determining their electronic, optical, mechanical and thermodynamic properties using DFT. The materials possess similar properties as cubic Br doped CsPbI3 perovskites. The mechanical properties of these compounds revealed that they are ductile in nature and mechanically stable. In summary, the thesis present a novel work on introduction of impurities into CsPbI3 perovskite material as well as compositional engineering to alter its electronic and optical properties for solar cells application.
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    Search for high-mass resonances in the Zgamma channel and Quality assurance of Scintillation detector modules of Tile Calorimeter Phase-I Upgrade of the ATLAS detector
    (University of the Witwatersrand, Johannesburg, 2024-09) Mokgatitswane, Gaogalalwe; Ruan, Xifeng; Solovyanov, Oleg; Mellado, Bruce
    This thesis presents a search for narrow, high-mass resonances decaying to a Z boson and a photon (Zy) in the final state. The analysis utilizes the full Run 2 dataset collected by the ATLAS experiment at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 140 fb-1 of proton-proton col- lisions at a center-of-mass energy of ps = 13 TeV. The search focuses on a mass range of 220 GeV and 3400 GeV, aiming to identify deviations from the expected background arising from Standard Model processes. A small excess is observed at 250 GeV within the area of interest, with a combined significance of 2.1 standard deviations, indicating the need for further investigation with more data. Upper limits are set on the production cross-section times branching ratio for resonances decaying to Zy across the investigated mass range. When considering spin-0 resonances produced through gluon-gluon fusion, the observed limits at a 95% confidence level range from 65.5 fb to 0.6 fb. For spin-2 resonances produced via gluon-gluon fusion (with quark-antiquark initial states), the limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb. The thesis also highlights the successful Phase-I upgrade of the Tile Calorimeter in the ATLAS detector, ensuring its continued performance. This involved the replacement of degraded Gap-Crack scintillators and Minimum Bias Trigger Scintillators (MBTS) with non-irradiated ones, along-side optimising their geometry, all in preparation for data taking during LHC Run 3. These upgrade endeavors encompassed the design of new Gap-Crack and MBTS counters, including extensions to higher rapidity, the assembly of these counters, their rigorous qualification, and characterization using radioactive sources (90Sr and 137Cs), along with their seamless integration onto the ATLAS detector.
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    Evaluation of radiation damage on lutetium-aluminium and gold for practical applications using proton irradiation as a surrogate for neutrons
    (University of the Witwatersrand, Johannesburg, 2024-10) Temaugee, Samuel Terungwa; Mavunda, Risimati D.; Usman, Iyabo T.
    An understanding of the interaction of energetic radiations from particles such as protons, neutrons, and photons, with the microstructure of materials is crucial for predicting their bulk morphological response in extreme radiation environments. Exposure to these radiation species could lead to changes in the microstructural properties that, in turn, affect the mechanical and physical properties of the macroscopic matter. This thesis investigated the resilience of materials, specifically Au and Lu-Al, to radiation damage, employing computational simulation methods and experimental techniques. The study aims to provide critical insights into the radiation damage sturdiness of Au and Lu-Al, considering their application in reactor technology and other extreme radiation environments. Monte Carlo-based methods were employed to calculate radiation damage in the samples resulting from neutron and proton irradiation, utilizing MCNP6.2 and SRIM-2013, respectively. The objective was to compare ion beam irradiation with neutrons with a view to utilizing proton irradiation as a surrogate for neutron irradiation. Three different state-of-the-art characterization techniques—X-ray diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM), and Flash Differential Calorimetry(F-DSC)—were employed to evaluate damage in the materials before and after proton irradiation using the CLASS Accelerator at MIT, USA. The results of the study indicated that protons within the energy range 0.1 to 1.0 MeV produced similar types of damage in the materials as would neutrons (spectrum 0< E≤20 MeV at SAFARI reactor), suggesting protons as an alternative to neutron irradiation. Defect characterization in the materials using XRD and HRTEM techniques revealed dislocation loops and lines in both Lu-Al and Au, as well as Stacking Faults Tetrahedra (SFT) in the Au material. These defects with proton irradiation were similar to those observed with neutron irradiation in Au and other aluminum alloys. The estimated defect number density ranged from 0.7 to 4.8 × 1014 m−2, showing an increase with rising displacements per atom (dpa) or proton fluence post-irradiation. Lu-Al exhibited higher defect density values than Au, consistent with Monte Carlo simulations. Furthermore, results from the Flash DSC technique revealed significant changes in the characteristics of the power-temperature profiles (melting curves) of Lu-Al as dpa increased, offering insights into radiation-induced processes such as phase transition and precipitate stability at specific defect annealing temperatures. These findings are crucial for radiation damage studies for the binary alloy and warrant further investigation. The observed microstructural defect densities were relatively high, and prolonged exposure of the materials to higher doses could lead to further changes in microstructural properties, consequently influencing the physical and mechanical properties of the macroscopic material.
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    Optimization of Prostate Plan in a Pelvic Prosthesis Phantom
    (University of the Witwatersrand, Johannesburg, 2024-09) Dumela, Khombo Eunice; Oderinde, Oluwaseyi M.; Usman, IyaboT.
    Background: An increasing number of elderly prostate cancer patients with high-density material hip prosthesis are referred for external beam Radiotherapy (EBRT). Radiation treatment of pelvis cancer patients with high-density hip prosthesis needs special attention because of the artifacts created in the computed tomography (CT) field of view and the radiotherapy dosimetry challenges. The accuracy of the treatment planning dose calculation algorithms determines the accuracy of the dose delivered to the patient during radiation therapy. However, the most available algorithms do not accurately model the absorption of high-density metals’ scattering properties and underestimate the resulting dose perturbations. Aim: This study aims to optimize the dose distribution of prostate 3D conformal treatment, intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) in an in-house metallic hip prosthesis phantom. Methods and materials: In this study, an ionization chamber and Gafchromic (EBT3) films were used to physically measure the prostate point dose in an in-house pelvic phantom. The pelvic phantom was irradiated on the Linac with four static fields, namely, (1) anterior field, (2) posterior field, (3) right lateral field passing through the bone of the normal hip and (4) left lateral passing through the hip prosthesis. IMRT and VMATs plans were also generated on the phantom. The phantom was also irradiated with IMRT and VMATs plan. The use of single arc versus two arcs with avoidance sector were also evaluated. The phantom consists of different materials; Nylon-12 (a solid water-equivalent material) to simulate the prostate with a central cavity to accommodate an ionization chamber and film, superflab gel bolus to simulate human soft tissue, dental wax to simulate human soft tissue, bone anatomy for the right hip and a titanium implant to replace the bony structure of the left hip. For the static fields, an in-house pelvic phantom was simulated using the EGSnrc Monte Carlo code, and 6 and 15 MV photon energies were employed as in an experimental setting. The prostate point doses computed by the Treatment Planning System (TPS), measured using ionisation chamber, and Gafchromic EBT3 film were compared with the prostate point doses simulated by Monte Carlo code. Results and discussion: The novel phantom was constructed using superflab gel bolus, Nylon-12, dental wax, pig bone insert and a titanium alloy hip replacement. The radiological equivalence of the superflab gel bolus and dental wax was determined employing linear attenuation coefficients and then compared to an RW3 Solid water phantom. EGSnrc Monte Carlo (MC) code was used in this study. Before using Monte Carlo codes, they need to be validated by comparing the Linear accelerator Monte Carlo simulated dose distribution with the experimental data measured in a Linear accelerator using water and ionization chamber for 6 MV and 15 MV photon beams of different field sizes. The EGSnrc dose distributions were compared with the experimental measurements using a gamma analysis, employing a 2 %/2 mm distance-to-agreement criterion. The EGSnrc Monte Carlo calculated dose distribution agreed well with experimental measurements within 2 %. The MC beam model was then used to compute the dose distribution in an in-house pelvic phantom. The comparison of the measurements between the TPS calculated prostate point dose and ionization chamber for the 6 MV and 15 MV photon beams was: anterior (gantry 0°) 1.8 % and -0.5 %; posterior (gantry 180°) 1.7 % and -0.2 %; left lateral (gantry 90°) 6.3% and 4.2 %; right lateral (gantry 270°) -2.2 % and -2.1 % respectively. Results obtained for Gafchromic EBT3 film measured doses were: anterior 2.3 % and 1.3 %; posterior -0.9 % and 0.2 %, left lateral 4.5 % and 3.5 %; right lateral -2.1 % and -2.5%, for the 6 MV and 15 MV photon beams, respectively. Consequently, results obtained for comparison of TPS, ion chamber and Film with MC simulated doses were: anterior 3.9 %, -2.1 and -1.6% %; posterior 1.8 %, -0.1% and -2.7 %; left lateral -0.2 %, 6.5 % and 4.7 %; right lateral 0.4 %, -2.6% and -2.5 %, for the 6 MV photon beam. And for 15 MV photon beam the results were: anterior 1.9 %, -3.8 and -0.6%; posterior 2.0 %, -2.3 % and -2.2 %; left lateral 0.5 %, 3.7 % and 2.9 %; right lateral 0.4 %, -2.4 % and -2.9 %. Monte Carlo simulations and film measurements have a statistically significant difference of p<0.001, with the film measurements having a higher value than MC simulations except on the left lateral field. Monte Carlo simulations and ionization chamber measurements also show a significant difference of p<0.001, with the ionization chamber having a higher value than the MC simulation, except for the left lateral field passing through the hip prosthesis. The comparison of the measurements between the TPS calculated prostate point dose with ionization chamber and Gafchromic EBT3 film for the 6 MV IMRT plan of the beam passing through the prosthesis was 2.2 % and 3.3%, respectively. While the IMRT plan with avoided beam was 1.9 % and 3.1% for ionization chamber and Gafchromic EBT3 film, respectively. The comparison of the measurements between the TPS calculated prostate point dose for the 6 MV VMAT plan without avoiding for the beam passing through the prosthesis was 1.1 % and 2.2 % for ionization chamber and Gafchromic EBT3 film, respectively. While for VMAT plan with avoided sector as 3.0 % and 4.0% for ionization chamber and Gafchromic EBT3 film, respectively. The test suggested a significant difference of p=0.0001 between the distribution of film measurements and TPS calculated dose. Meanwhile, for ionization chamber measurements and TPS calculated dose; the test indicated a significant difference between ion chamber measurements and TPS calculated dose with a significant level of less than 0.001. in addition, MC simulated dose and TPS calculated dose; the test shows a percentage difference of -0.2 % and 0.5 % for 6 MV and 15 MV photon beams in the lateral field that passes through the prosthesis. The test indicated the significant difference of p=0.001 which is slightly lower compared to the other comparisons. Conclusion: The dual dosimetric pelvic prosthesis phantom is easy to assembly and is more convenient for second dose check for patients with hip prostheses. Through the use of the pelvic phantom, it was possible to measure the prostate point dose using ionization chamber and films. The TPS overestimated the prostate point dose because the treatment planning algorithm could not accurately determine the CT number and the electron density of the prosthesis due to the limitation on the CT scanner. The maximum deviation calculated in this study for TPS, ionization chamber Gafchromic EBT3 films when compared to Monte Carlo simulated dose comes from the lateral fields passing through the prosthesis for both 6 MV and 15 MV photon beams.
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    Surface Brillouin scattering studies of high-temperature elasticity
    (University of the Witwatersrand, Johannesburg, 1999-03) Stoddart, Paul Randall; Comins, J. Darrel
    A novel technique has been developed for studying the elastic proper­ ties of opaque solids at high temperatures. The method is based on surface Brillouin scattering (SBS) and has the advantages of being contact-free and non-destructive. The elastic constants can be extracted from SBS measure­ments of the directional dependence of the surface wave velocities. An optical furnace was designed to provide the special scattering geometry required for these measurements. The technique has been evaluated on silicon and a single-crystal nickel-based superalloy, with measurements up to 800°C and 200°C respectively. Above these temperatures, measurements were precluded by a marked deterioration in the surface quality. The elastic constants for sil­icon compare favourably with the established ultrasonic values, particularly in terms of the changes as a function of temperature. Additional measure­ment were performed on silicon at temperatures up to 900°C in order to examine the well-known central mode feature. These results shed light on a major outstanding problem in SBS, because they reveal the presence of a second quasielastic mode that may be associated with scattering from dif­fusive excitations. Further measurements at high and low temperatures are proposed to confirm the mechanism. Silicon was also used as a test system to clarify certain aspects of the theory and practice of SBS that have not been properly dealt with before, such as the effects of surface anisotropy and of the extended collection aperture. This indicates that SBS provides effective elastic constants for the outer 300 nm of the sample surface and thus may be influenced by surface damage and surface contamination. In the case of the superalloy, the difficulties encountered in gathering data at higher temper­atures suggests that modifications to the furnace arrangement are required. The larger relative error in the velocities also created problems in the extraction of the elastic constants. This difficulty was satisfactorily overcome by using the longitudinal threshold in the Lamb shoulder to fix the value of c₁₁. Although the work described here has been limited to temperatures below 900°C, it is clear that SBS provides a powerful method for probing the elastic properties of opaque solids at elevated temperatures.